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Clinical outcomes remain heterogeneous among patients with unresectable stage III non-small cell lung cancer treated with definitive chemoradiotherapy. We evaluated the prognostic value of the systemic immune-inflammation index (SII) and explored survival outcomes according to the concurrent chemotherapy backbone in this setting. This retrospective study included 101 patients treated with definitive chemoradiotherapy before consolidation durvalumab became standard practice. Baseline SII was calculated using routine blood counts. Overall survival (OS) and progression-free survival (PFS) were analyzed using the Kaplan-Meier method and Cox proportional hazards models. With a median follow-up of 40.6 months, patients with low SII had significantly longer OS (26.1 vs. 10.9 months, p = 0.004) and PFS (15.6 vs. 7.2 months, p = 0.018) than those with high SII. Elevated SII remained independently associated with inferior OS in multivariable analysis (HR 1.99, 95% CI 1.21-3.27, p = 0.007). Although survival was numerically longer with cisplatin/etoposide than with carboplatin/paclitaxel, the difference was not statistically significant. Subgroup analysis according to SII level and chemotherapy regimen showed the poorest outcomes among patients with high SII receiving carboplatin/paclitaxel. These findings support the prognostic value of SII and suggest a potential role for risk stratification in patients with unresectable stage III NSCLC undergoing definitive chemoradiotherapy in the pre-durvalumab era.